Process for thermal hydrodealkylation



United States Patent 3,284,526 PROCESS FOR THERMAL HYDRODE- ALKYLATIONJames A. Frayer, Pittsburgh, Robert F. Mansfield, New Kensington, RodneyE. Peterson, Oakmont, and Eldon M. Sutphin, Pittsburgh, Pa., assignorsto Gulf Research & Development Company, Pittsburgh, Pa., a corporationof Delaware No Drawing. Filed Sept. 4, 1963, Ser. No. 306,598 9 Claims.(Cl. 260672) This invention relates to a process for the thermalhydrodealkylation of alkyl aromatics, particularly to a process for thethermal hydrodealkylation of alkyl aromatics without the formation ofappreciable amounts of carbon or coke deposits.

Alkyl aromatics, such as toluene, can be dealkylated to lighteraromatics, such as benzene, by subjecting such alkyl aromatic in thepresence of hydrogen to an elevated temperature and an elevated pressurefor a controlled length of time. As a result of such reaction conditionsthe alkyl group is cleaved from the alkyl aromatic and combines with thehydrogen present to form a saturated aliphatic hydrocarbon. The desiredaromatic can be separated from the saturated aliphatic hydrocarbon andunreacted alkyl aromatic, if present, in any convenient way.

In order to effect the desired thermal hydrodealkylation of the alkylaromatic, it is imperative that the temperature of the mixture of alkylaromatic and hydrogen during such reaction be maintained within a rangeof about 1150 to about 1800 F., preferably about 1250 to about 1350 F.for about one to about 400 seconds, preferably about 10 to about 100seconds. The initial step in the process, therefore, involves heatingthe mixture of alkyl aromatic and hydrogen to the reaction temperature.Sufificient hydro-gen must be present to replace the alkyl chain cleavedfrom the aromatic ring at the elevated reaction temperatures and also tocombine with the alkyl chain to form therewith a saturated aliphatichydrocarbon. In the general molar ratio of hydrogen to alkyl, aromaticcharge can be from about 1.5 to about 20, preferably from about three toabout eight. Any alkyl aromatic can be so treated, for example, toluene,xylenes, tri methyl benzene isomers, alkyl nap'hthalenes and mixturesthereof, alkyl phenols, etc. There is a tendency, however, during thepreheating step, particularly in start-up procedures, for the alkylaromatic to be converted to the corresponding saturated compound.Toluene, for example, has a tendency during the initial heating periodto be converted to methylcyclohexane. When these saturated materials arethen introduced into the hydrodealkylation zone, wherein the elevatedtemperature defined above exists, they tend, in the presence ofhydrogen, to crack, forming both carbon, or coke, and gases which arepredominantly methane. Such conditions lead to lower yields of thedesired dealkylated aromatic and require frequent shutdowns because ofcoke deposits in the thermal hydrodealkylation reaction zone.

We have found that the above difficulties can be avoided and the desiredthermal hydrodealkylation of alkyl aromatics can be effected with littleor no coke iorrnation or undesirable methane production by maintainingthe mixture of alkyl aromatic and hydrogen during the preheating periodat a temperature below about 85 0 to about 950 F., preferably belowabout 900 F., for a time no greater than about 50 seconds, preferablyfor about one to about 10 seconds. Contact or residence time herein isdefined as the length of time a differential volume of material remainsin the preheating zone or the thermal hydrodealkylation zone at thedefined temperature. Under such conditions the charge alkyl aromatic isnot saturated to any appreciable extent during the preheating period andthe only substantial reaction occurring during hydrodealkylation periodis the desired hydrodealkylation of the alkyl aromatic.

This procedure is particularly applicable during the start-up. Forexample, initially hydrogen can be passed through the system in a closedcycle, and suitable heat can be added thereto in the preheating zone toraise the temperature of the system to a defined temperature level, forexample, about 1000 to about ll00 F. Cold alkyl aromatic is thenintroduced into the preheating zone with the hot circulating hydrogen.Since the addition of cold alkyl aromatic to the pre'heater will have atendency to lower the temperature level thus obtained in the preheatingzone, additional heat is added thereto in order to quickly raise thetemperature therein again to the defined level. This procedure willavoid excessive residence time of the reactants at the undesired lowtemperature level and thereby eliminate the defined hydrogenation in thepreheating zone and the cracking of the saturated ring compounds in thethermal hydrodealkylation zone to form undesirable gases and coke.

The mixture of hydrogen and alkyl aromatic when introduced into thepreheating zone is at a temperature of about to about 800 F., preferablyat a temperature of about 700 to about 800 F. When the mixture of alkylaromatic and hydrogen is at a temperature above about 75 F. when it isintroduced into the preheating zone, it can be heated to suchtemperature in any convenient manner, for example, by indirect heatexchange relationship with the heated products from the thermalhydrodealkylation reaction zone. In any event the time the reactionmixture is maintained above about 75 F., but below about 850 to about950 F., preferably below about 900 F. during the defined preheatingperiod, whether all or only a portion of the heat acquired by saidmixture was obtained in the preheating zone, must be about one to about10 seconds, but in no event more than about 50 seconds.

The preheating zone can be heated in any suitable manner, for exampleindirectly by gas-fired heaters. The mixture leaving the preheating zoneis at a temperature of about 1150 to about 1250 F. Therefore thetemperature therein can range from a low of about 70 F. to a high ofabout 125 0 F., but the average temperature in the preheating zone willbe from about 1000 to about 1100 F., preferably about 1050 F. Underthese conditions no more than about five percent, and generally lessthan about two percent, by Weight of the alkyl aromatic is dealkylatedtherein.

The heated mixture is then introduced into the hydrodealkylation zone,at which point the desired hydrodealkylation reaction takes place. Sincethis reaction is exothermic it is accompanied with release of heat. Thetemperature can be maintained therein at any temperature level in anysuitable manner or the heat resulting from hydrodealkylation can bepermitted in large measure to remain therein, resulting in anappreciable temperature rise of the products in the hydrodealkylationreaction zone. In any event the temperature in the hydrodealkylationreaction zone will remain Within a range of about 1150 to about 1800 F.,preferably about 1250 to about 1350 F. The residence period Will beabout one to about 400 seconds, preferably about 10 to about seconds.

The pressure drop throughout the system will not be appreciable andtherefore the pressure will be substantially the same throughout. Thusthe pressure can be from about 100 to about 1000 pounds per square inchgauge, preferably about 400 to about 600 pounds per square inch gauge.

Upon removal from the hydrodealkylation reaction zone, the productresulting from the reaction therein will comprise principallydealkylated alkyl aromatic, unreacted charge aromatic, methane andexcess or unreacted hydrogen. In a matter of about one to about fiveseconds, for example, the reaction mixture is cooled by any convenientmeans, for example, by indirect heat exchange relationship with freshalkyl aromatic and hydrogen charge, to a temperature below about 600 F.and after further cooling through heat exchangers to ambienttemperature. Hydrogen and other gases are then vented from the reactionmixture and the remainder is separated into its component parts by anysuitable means, preferably by distillation at a temperature of about 175to about 250 F. and a pressure of about one to about 10 pounds persquare inch gauge.

The invention can further be illustrated by the following. Four runswere made on a mixture of toluene and hydrogen wherein the temperatureof the preheater and the hydrodealkylation reactor were varied and theresidence times in the preheater were also varied. The results obtainedare set forth below in the table. The mixture of toluene and hydrogenwas in each run introduced into the preheater at a temperature of 75 F.

Table Run No 1 2 3 4 Operating Conditions:

Average PreheaterTemperature,

700 815 955 1, 175 Average Reactor Temperature,

600 1, 280 1, 277 1, 280 Unit Pressure, Pounds per Square Inch Gauge 460460 460 460 Hydrogen to Toluene Mole Ratio 8. 8. 0 8. 0 4. 0 ContactTime in Preheater,

Seconds 166 151 136 6 Contact Time in Reactor, Seconds 113 68 68 50Product Gas Composition: Mole Percent:

Hydrogen-- 41. 4 16. 3 51. 4 57. 1 Methane" 53. 2 75. 4 46. 7 41. 4Ethane"- 1.7 7. 2 1.8 1 O to O 3.7 1.1 0.0 Yield Oi Total LiquidProduct:

Percent By Volume 01 Charge... 68. 2 24. 5 51.1 83. 5 Inspection onTotal Liquid Product:

Gravity, API 55.1 27.8 28.6 28.0 Analysis, Percent by Volume of urge:

l2. 8 23. 7 1. 7 20.0 10. 3 1. 8 0.8 0.2 0. 0 1. 0 0.0 0.5 0.0 0.2saturates 54. 4 0.8 Methylcyclohexane 33. 2 0.5 Cyclohexane 5. 6 0.2Methylcyclopentane. 2. 3 Trace Isopentane. 2. 2 Trace Other 0 -01 10. 70.1 Olefins 1.0 0.0 Aromatic Ring Balance:

Percent Aromatic Rings Recovered 11. 4 28. 6 00. 0 98.6 Percent RingsLost Due to Saturation and/or Cracking 88. 6 71. 4 40. O 1. 4

A study of the data in the table illustrates the advantages of operatingin accordance with the process defined and claimed herein. In run No. 1the average preheater temperature was 700 F. and the reactor temperaturewas 600 F. In the second and third runs the reactor temperature wasessentially the same, 1280" F. and 1277" F., respectively, but thepreheater temperature in run No. 2 was 815 F. and in run No. 3 955 F.The contact time in the preheater in the first three runs wasapproximately the same, 166, 151 and 136 seconds, respectively.

In run No. 1 the yield of saturates was 54.4 percent by volume, of which33.2 percent was methylcyclohexane. Very little hydrodealkylationoccured in the hydrodealkylation reactor. Very little aromatics wererecovered, and 88.6 percent were lost due to saturation and/or cracking.In run No. 2 saturation was again occurring in the preheater, but thesaturated compounds so formed were at the elevated temperature existingin the hydrodealkylation reactor cracked to methane and ethane. Somebenzene was produced. Hydrogen consumption was high, since saturation,cracking and hydrodealkylation were all occurring and consuming the samein run No. 2. The percent rings lost due to saturation and/ or crackingwas still high. Merely raising the preheater temperature in run No. 3but reducing only slightly the contact time in the preheater wassufiicient to improve the operation but not satisfactorily. The APIgravity of the liquid product indicates that the product is essentiallybenzene. However, only 60 percent of the benzene rings were recoveredand 40 percent of the rings were lost due to saturation and/or cracking.Operation in accordance with the process of the invention as exemplifiedby run No. 4 shows that most of the toluene went to benzene. Only 1.4percent of the rings were lost due to saturation and/or cracking.

Obviously many modifications and variations of the invention, ashereinabove set forth, can be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

1. A process for the hydrodealkylation of an alkyl aromatic whichcomprises heating a mixture of said alkyl aromatic and hydrogen to atemperature of about 1150 to about 1250 F., maintaining the temperatureof said mixture during said heating period at a temperature below about950 F. for a time less than about 50 seconds, and thereafter subjectingthe resultant heated mixture to a temperature of about 1150 to about1800" F. and a pressure of at least about pounds per square inch gaugefor about one to about 400 seconds to effect dealkylation of said alkylaromatic.

2. A process for the hydrodealkylation of an alkyl aromatic whichcomprises heating a mixture of said alkyl aromatic and hydrogen to atemperature of about 1150 to about 1250 F., maintaining the temperatureof said mixture during said heating period at a temperature below about950 F. for about one to about 10 seconds, and thereafter subjecting theresultant heated mixture to a temperature of about 1150 to about 1800 F.and a pressure of at least about 100 pounds per square inch gauge forabout one to about 400 seconds to effect dealkylation of said alkylaromatic.

3. A process for the hydrodealkylation of toluene which comprisesheating a mixture of said toluene and hydrogen to a temperature of about1150 to about 1250 F., maintaining the temperature of said mixtureduring said heating period at a temperature below about 950 F. for atime less than about 50 seconds, and thereafter subjecting the resultantheated mixture to a temperature of about 1150 to about 1800 F. and apressure of at least about 100 pounds per square inch gauge for aboutone to about 400 seconds to etfect dealkylation of said toluene.

4. A process for the hydrodealkylation of toluene which comprisesheating a mixture of said toluene and hydrogen to a temperature of about1150 to about 1250 F., maintaining the temperature of said mixtureduring said heating period at a temperature below about 950 F. for aboutone to about 10 seconds, and thereafter subjecting the resultant heatedmixture to a temperature of about 1150 to about 1800 F. and a pressureof at least about 100 pounds per square inch gauge for about one toabout 400 seconds to effect dealkylation of said toluene.

5. A process for the hydrodealkylation of an alkyl aromatic whichcomprises heating a mixture of said alkyl aromatic and hydrogen to atemperature of about 1150" to about 1250 F., maintaining the temperatureof said mixture during said heating period at a temperature below about850 F. for a time less than about 50 seconds, and thereafter subjectingthe resultant heated mixture to a temperature of about 1250 F. to about1350 F. and a pressure of at least about 100 pounds per square inchgauge for about 10 to about 100 seconds to effect dealkylation of saidalkyl aromatic.

6. A process for the hydrodealkylation of an alkyl aromatic whichcomprises heating a mixture of said alkyl aromatic and hydrogen to atemperature of about 1150 to about 1250 F., maintaining the temperatureof said mixture during said heating period at a temperature below about850 F. for about one to about 10 seconds, and thereafter subjecting theresultant heated mixture to a temperature of about 1250 to about 1350 F.and a 10 pressure of at least about 100 pounds per square inch gauge forabout 10 to about 100 seconds to effect dealkylation of said alkylaromatic.

7. A process for the hydrodealkylation of toluene which comprisesheating a mixture of said toluene and hydrogen to a temperature of aboutll50 to about 1250 F., maintaining the temperature of said mixtureduring said heating period at a temperature below about 850 F. for atime less than about 50 seconds, and thereafter subjecting the resultantheated mixture to a temperature of about 1250 F. to about 1350 F. and apressure of at least about 100 pounds per square inch gauge for about 10to about 100 seconds to effect dealkylation of said toluene.

8. A process for the hydrodealkylation of toluene which comprisesheating a mixture of said toluene and hydrogen 25 to a temperature ofabout 1150 to about 1250" F., maintaining the temperature of saidmixture during said heating period at a temperature below about 850 F.for about one to about 10 seconds, and thereafter subjecting theresultant heated mixture to a temperature of about 1250 F. to about 1350F. and a pressure of at least about 100 pounds per square inch gauge forabout 10 to about 100 seconds to effect dealkylation of said toluene.

9. A process for the hydrodealkylation of toluene which comprisesheating a mixture of toluene and hydrogen to a temperature of about 1175F. in less than about six seconds and thereafter subjecting theresultant heated mixture to a temperature of about 1280 F. and apressure of about 460 pounds per square inch gauge for about 50 secondsto effect dealkylation of said toluene.

References Cited by the Examiner UNITED STATES PATENTS 3,193,595 7/1965Kenton et al 260672 FOREIGN PATENTS 790,595 2/ 1958 Great Britain.

OTHER REFERENCES Silsby et al.: Journal of Applied Chemistry, vol. 6,August 1956, pp. 347-356.

DELBERT E. GANTZ, Primary Examiner.

C. R. DAVIS, Assistant Examiner.

1. A PROCESS FOR THE HYDRODEALKYLATION OF AN ALKYL AROMATIC WHICHCOMPRISES HEATING A MIXTURE ON SAID ALKUL AROMATIC AND HYDROGEN TO ATEMPERATURE OF ABOUT 1150* TO ABOUT 1250* F., MAINTAINING THETEMPERATURE BELOW MIXTURE DURING SAID HEATING PERIOD AT A TEMPERATUREBELOW ABOUT 950* F. FOR A TIME LESS THAAN ABOUT 50 SECONDS, ANDTHEREAFTER SUBJECTING THE RESULTANT HEATED MIXTURE TO A TEMPERATURE OFABOUT 1150* TO ABOUT 1800* F. AND A PRESSURE OF AT LEAST ABOUT 100POUNDS PER SQUARE INCH GAUGE FOR ABOUT ONE TO ABOUT 400 SECONDS TOEFFECT DEALKYLATION OF SAID ALKYL AROMATIC.